Country suffering from major fuel shortages with oil refineries targeted by striking oil workers.

France is suffering from major fuel shortages, with oil refineries blockaded by striking oil workers who are unhappy with planned changes to France's labour regulations.

Six out of the eight refineries in the country have either stopped operating or have reduced output.

Riot police fired tear gas and water canon to break up a picket line blocking access to Exxon Mobil Corp's refinery outside the southern port city of Marseilles, as scores of petrol stations nationwide ran out of fuel on Tuesday.

"Enough is enough," said Prime Minister Manuel Valls.

The pre-dawn swoop drew a sharp riposte from the Confederation Generale du Travail (CGT) union, which wants to force President Francois Hollande's government to rethink the labour reforms designed to make it easier for companies to hire and fire employees.

The CGT described the police operation as an act of "unprecedented violence" as it and other unions served notice of a June 3-5 strike by air traffic controllers that will dovetail with walkouts by state rail employees, port workers and staff on the Paris metro and suburban rail networks.

Hollande, on the other hand, says labour reform is crucial to creating new jobs and blames the fuel blockade on a small minority of people.

In parts of France, motorists swamped petrol stations, some using real-time mobile phone applications to learn where they could refill without queuing for hours or being turned away.

"There is no petrol anywhere," one motorist in Paris said.

"I'm upset because we seem to have been taken hostage by this strike. I understand why they must go on strike and why we must respect it, but for people who work, this is not practical," she said.

The panic-buying amid fuel rationing in parts of France will compound the troubles of Hollande and his government as they strive to convince voters that "things are getting better".

Emergency stocks could keep fuel stations open for up to two months if tapped, according to experts. But Valls said the actions of pickets aimed at halting distribution would not be tolerated.

Hollande, France's most unpopular leader in recent memory, has said he will only run for re-election if he hauls the jobless rate down.

The International Monetary Fund said this week that France's economy was not recovering quickly enough to cut unemployment and debt significantly, and would not do so without further reforms.